The present invention relates to sensing devices. More particularly, the present invention relates to monolithically integrated, compound semiconductor sensing devices and methods of manufacturing such devices.
Sensing devices are used in various applications, some of which subject these sensing devices to harsh operating conditions such as temperature extremes, dirty environments and electromagnetically active surroundings. For example, in the automobile industry sensing devices are used in crankshaft position detection, wheel speed sensors, brushless electric motors, and other environments subject to wide temperature variations. Automotive sensor specifications require sensors that continually operate with significant change in performance across temperatures ranging from xe2x88x9240xc2x0 C. to over 200xc2x0 C.
One conventional sensing device is a magnetic field sensing device. A magnetic field sensing device can be fabricated using a silicon semiconductor Hall sensor that is typically integrated into a silicon semiconductor signal conditioning circuit. A signal conditioning circuit is used to convert the output signal from the Hall sensor into a signal that is useful for its intended application.
Silicon Hall sensors possess the disadvantages of lower sensitivity and signal-to-noise ratio because of the low electron mobility of silicon. Accordingly, these silicon devices lack precision and require demanding signal conditioning circuitry. Additionally, traditional silicon circuitry is not suitable for use over a wide temperature range, particularly at temperatures greater than about 150xc2x0 C. At higher temperatures, issues such as leakage current and parasitic conductance significantly impact the operation of silicon circuitry.
Sensing devices utilizing compound semiconductor technology are also available. Manufacture of these devices, however, involves separate fabrication of the integrated circuit components. In a standard manufacturing process, a magnetic field sensor, such as a Hall sensor, a magnetoresistor or a magnetotransistor, is fabricated by depositing at least one layer, usually multiple layers, of compound semiconductor material such as InSb on a substrate material. Typically, each of the layers used to form the magnetic field sensor has a different composition or contains a different dopant or dopant level than the adjacent layers. The materials used to form the layers in the manufacture of magnetic field sensors may be deposited as thin epitaxial films utilizing a process such as metal organic chemical vapor deposition (MOCVD), vapor phase epitaxy (VPE), liquid phase epitaxy (LPE) or molecular beam epitaxy (MBE). In a separate process, an application specific integrated circuit (ASIC) to provide signal conditioning is manufactured.
A prior art method for manufacturing a sensing device is shown in FIGS. 1A-1D. Referring to FIG. 1A, a substrate 10 having a top surface 12 may comprise a compound semiconductor material such as semi-insulating GaAs or other suitable III-V compound semiconductor material. Referring now to FIG. 1B, a sensor element 14 is shown on the surface 12 of the substrate 10. The sensor element may be any conventional sensor element, including a magnetic sensor, such as a Hall effect or magnetoresistive type sensor. Such a structure can be formed by one or more epitaxial layers. As shown in FIG. 1C, a separate substrate 20 is provided for manufacture of a signal conditioning circuit, which may be fabricated using either silicon or compound semiconductor technology.
FIG. 1D shows an example of signal conditioning circuit layers incorporating a heterojunction bipolar transistor (HBT) structure 22 deposited on substrate 20. As is known in the art, an HBT structure may include active layers such as an emitter layer 24, a base layer 26 and a collector layer 28. These layers may be etched or otherwise treated in conventional ways to form the elements of the signal conditioning circuit. Additional layers, such as dielectric layers and metallization layers (not shown), may be formed to provide contacts for interconnection with another device, and to interconnect elements of the signal conditioning circuit with one another. After separate fabrication of the magnetic sensor element and the signal conditioning circuit, the two structures are interconnected by wirebonding the structures together to form a completed sensing device.
Separate fabrication of the sensor and the signal conditioning circuit has several drawbacks. Wirebonding uses valuable space on both the sensor and signal conditioning circuit. Wirebonded devices tend to exhibit lower reliability than integrated devices that do not require wirebonding of the separate elements. In addition, the wire-bonding process is labor-intensive, costly and increases the size of the finished assembly.
It would be useful to provide a sensing device capable of operating over a wider temperature range than devices fabricated using silicon semiconductor signal conditioning circuits. It would also be advantageous to provide a process for manufacturing a compound semiconductor sensing device having a sensor and signal conditioning circuit that is less labor intensive, more reliable and less expensive than methods that involve separate device fabrication and interconnection.
Accordingly, one aspect of the present invention provides a monolithically integrated, compound semiconductor sensing device including a signal conditioning circuit and a sensor element. In one embodiment, a sensor element including one or more compound semiconductors is epitaxially deposited over a first portion of the substrate surface. In another aspect of the invention, the semiconductor layers which will be used to form the signal conditioning circuit (the xe2x80x9csignal conditioning epitaxyxe2x80x9d) is formed by epitaxial deposition of one or more compound semiconductors on second portion of the substrate surface.
In a preferred embodiment, the signal conditioning epitaxy include one or more III-V compound semiconductor. Similarly, the sensor element includes at least one III-V compound semiconductor, which typically is different from the compound semiconductor used in the signal conditioning epitaxy. Preferably, the surface of the substrate is formed from GaAs.
In a preferred embodiment, the sensor is a magnetic sensor, such as a Hall effect or magnetoresistive sensor. In one aspect of the invention, the sensor is in direct contact with the substrate surface. In another aspect of the invention, a buffer layer may be disposed between the substrate surface and the sensor. Similarly, a buffer layer may be disposed between the signal conditioning epitaxy and the substrate surface.
Another aspect of the invention involves a method of manufacturing a monolithic compound semiconductor sensing device. The method includes forming signal conditioning epitaxy adapted to form a signal conditioning circuit on a substrate surface and providing a well in the signal conditioning epitaxy to expose the substrate surface. In another aspect of the invention, a sensor is formed in the well structure.
In one embodiment of the invention, formation of the well structure within the signal conditioning circuit involves etching the well within the signal conditioning epitaxy after deposition of the signal conditioning epitaxy. According to a further embodiment of the invention, formation of the well within the signal conditioning circuit involves masking the substrate surface, depositing the signal conditioning epitaxy over the mask and removing the mask to provide a well.
In still another embodiment of a the invention, a method of manufacturing a monolithic compound semiconductor sensing device is provided which includes forming a sensor on a substrate surface and providing a well in the sensor to expose the substrate surface. A signal conditioning epitaxy is formed within the well formed in the sensor.
According to one embodiment of the invention, the step of providing a well in the sensor includes etching the well in the sensor. In a further embodiment of the invention, the step of providing a well in the sensor epitaxy includes masking the substrate surface, depositing the sensor over the mask and removing the mask to provide a well.
Preferably, the method of the present invention involves forming the signal conditioning epitaxy first, and then later epitaxially depositing the sensor in a well formed in the signal conditioning circuit. In general, the materials used to form sensor epitaxy, such as InSb, are not stable at the high temperatures required to form the signal conditioning epitaxy. Therefore, it is preferred that the signal conditioning epitaxy is formed first, and the sensor element is formed after deposition of the signal conditioning epitaxy.
The methods according to this aspect of the invention preferably further include the step of forming all or part of a signal conditioning circuit from the signal conditioning epitaxy. Most preferably, this step includes forming connections on the monolithic structure between the sensor element and the signal conditioning circuit.
Compared to devices using silicon sensors, the integrated devices of the present invention provide higher sensitivity, improved signal-to-noise ratio, greater robustness against electrostatic discharge, and substantially no switching noise. The present invention also has the advantage of greater flexibility in providing features to the magnetic field sensing device. This signal conditioning circuit can include essentially any elements which can be used in such a circuit, such as, for example, amplifiers, power supply components such as voltage regulators and overvoltage protection components such as zener diodes. Monolithically integrating the sensor and the signal conditioning circuit in one die provides several other advantages.
The monolithically integrated manufacturing method of the present invention provides a simpler and less expensive method of manufacturing sensing devices which utilize a sensor and a signal conditioning circuit than conventional processes involving separate fabrication of the components. For example, the pick-and-place portion of the device assembly operation is simplified since only one die incorporating both components requires handling. The wire-bonding of the signal conditioning and sensor components is less expensive, due to the fact that only the input and output connections of the components need to be wired. In the conventional process, in which the components are separately fabricated, die-to-die interconnections also need to be separately wire-bonded.
It will be understood that fewer manufacturing steps provides an opportunity for improved product yields and reduced manufacturing errors. In addition, device reliability is improved due to the fact that the finished device has fewer interconnections. Additional features and advantages of the invention will be set forth in the following detailed description. It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed.